Field of the Invention
The present invention relates to a granule comprising a thermoplastic elastomeric polyurethane and bitumen, to the process for preparing it in an extruder and to a process for preparing a bituminous binder using said granule, which may especially be used for producing prefabricated sealing membranes by depositing said bituminous binder onto a fibrous support.
Description of Related Art
Membranes based on bitumen modified with polymers of the SBS (styrene-butadiene-styrene) or APP (atactic polypropylene) type have been known for many years, and have been widely developed since the 1970s in the sector of leakproofing terrace roofs.
Modification of direct distillation bitumens obtained from refineries is a necessity. Indeed, standard bitumens, used especially in the road industry, have softening points at temperatures of about 40° C. and are as hard and brittle as glass for temperatures typically below −5° C.
Now, these working temperatures are too limiting for use on roofs, where the temperature range may vary between −40° C. and 110° C. depending on the climate.
It is thus necessary to modify the bitumen. The standard techniques consist in adding to the bitumen thermoplastic polymers of block elastomer type [SBS, SEBS (styrene-ethylene-butadiene-styrene)] or plastomers [polyolefin such as APP, EVA (ethylene-vinyl acetate), IPP (isotactic polypropylene) or AAPO (amorphous alpha-polyolefin)].
Thus, via SBS modification, it is possible to obtain a bituminous binder with a softening point of about 125° C. and flexibility at low temperatures of about −30° C. The working range of a binder modified with plastomeric polymers is generally between −15° C. and 150° C.
However, membranes prepared with a bituminous binder modified with the abovementioned polymers have their limits:                SBS polymers are UV-sensitive and, as a result, the final layer (apparent layer) requires additional specific protection against UV attack;        plastomeric polymers have mediocre flexibility at low temperatures and the membrane seals produced with binders containing these polymers do not exhibit satisfactory heat-resistance.        
The additional specific protection against UV attack, when it is integrated into the prefabricated membrane, is currently produced either with metal strips (aluminum, copper or stainless steel) or with slate or mica flakes or granules sprinkled onto the surface during the manufacturing process. These protections called “self-protections” lead to an excess weight that may be up to 1.5 kg/m2, representing up to 30% of the weight of the membrane and increase the cost price of the final membrane.
Besides UV, these abovementioned known products age rapidly relative to the lifetime of the structures onto which they are applied and degrade by oxidation, preventing valid leakproofing over a long period. Restoring the leakproofing is in fact performed on average every 15-20 years.
Polyurethane modified bituminous coatings have been developed in recent years. These products especially find an application in road coverings and the leakproofing of artworks and buildings. Known products of this type are either:                cold-applied: the product is in liquid one-pack or two-pack form, and acquires its characteristics after the crosslinking which takes place in the open air;        hot-applied: the polymerization-crosslinking takes place in the hot bitumen just before its application on the worksite.        
For example, French patent application No. 2 064 750 in the name of NAPHTACHIMIE describes a thermoplastic bituminous binder containing less than 10% by weight of thermosetting polyurethane. The polyols used for synthesizing the polyurethane have functionalities of greater than 2, which may be up to 8, and the NCO/OH ratio is between 1 and 2 and preferably in the region of 1.1. This bituminous binder has thermoplastic properties due to the low proportion of polyurethane and may be applied onto a fibrous support to make a prefabricated membrane. However, the low percentage of polyurethane introduced does not make it possible to obtain membranes that are sufficiently elastic and resistant.
Moreover, European patent application No. 1 013 716 in the name of the Applicant discloses a one-pack liquid composition based on bitumen and polyurethane prepolymer which polymerizes directly on the support to be rendered leaktight when it is cold-applied on the worksite. Thus, this bituminous composition is not thermoplastic and is not intended for the preparation of prefabricated membranes in a factory.
Patent application WO 97/03253 in the name of INTERFACE, INC. discloses a sheet material intended to form a barrier against moisture and comprising a layer of a bitumen/polyurethane mixture protected with a polymer film and covered with a detachable film. The polyurethane used in the bituminous mixture is prepared with a polybutadiene polyol having a functionality of between 2.2 and 2.6. Thus, the resulting polyurethane is thermosetting and sensitive, by virtue of its chemical nature, to UV-mediated degradation, hence the need to protect the bituminous layer with a polymer film. The material of this patent application is not thermoplastic and it is cold-applied to the surface to be rendered leaktight and held in place by the adhesive power of the bituminous composition, optionally improved by the addition of a particular adhesive agent.
The Applicant described, in French patent application No. 2 844 283, a prefabricated sealing membrane based on a bituminous binder modified with a thermoplastic polyurethane having the following formulation, the percentages being weight percentages based on the total weight of the composition:                40% to 90% of bitumen,        10% to 50% of thermoplastic polyurethane,        0 to 10% of aromatic oil,        0 to 50% of fillers, and        0 to 0.5% of catalyst.        
The process for preparing the bituminous binder consists in synthesizing the thermoplastic polyurethane in situ in the hot bitumen. The obtained bituminous binder is then applied as a layer onto a fibrous support, for example by coating and then calendering, to give a prefabricated sealing membrane that may especially be used for leakproofing roofs.
However, the in situ polymerization of the polyurethane in hot bitumen described in this patent application is complicated from an industrial viewpoint. Specifically, certain bitumens cannot be used since they are incompatible with the synthesized polyurethanes. Furthermore, the bitumen must necessarily be heated in order to become liquid and the polyurethane bond formed during polymerization is reversible at temperatures of about 150° C., which degrades the quality of the polyurethane. Furthermore, the viscosity of the constituents requires proceeding via an additional step of synthesizing a quasi prepolymer. Finally, once the in situ synthesis is complete, the final viscosity of the bituminous binder is high and requires specific pumps.